I can attest to what the others are saying from the point of the secondary. I entered into a triad situation with a couple who *thought* they were rock solid. I had no reason to disbelieve them. And they themselves believed it. However, the addition of myself into their mix exposed the cracks in what they thought was a solid foundation. They are still working through the shit, and I am on my own again.
From my perspective, it was very painful to ultimately be rejected in spite of understanding why it had to be that way, and it's been very painful to see how hurt they both are, and that it may take years for them to rebuild what they *thought* they had. (I keep thinking, hell if we aren't going to be together, then I would feel better if at least they were happy. But they aren't. Worst possible outcome imo.)
So when you're talking about bringing other people into your shit ~ shit you are well aware of (unlike my couple) ~ then you've got to realize what you may be subjecting the outside party to.